The present invention relates in general to electrical impedance tomography and in particular, to a new and useful method for obtaining sets of current patterns which are used for producing a three-dimensional impedance image of the interior of a body.
The present invention has applications in medical imaging, clinical monitoring, non-destructive evaluation, process control and the imaging of multiphase fluid flow.
Although superficially similar to X-ray computed tomography or positron emission tomography, electrical impedance tomography (EIT) encounters fundamentally different problems when attempting to create an image. In X-ray computer tomography, for example, the paths of photons through the body are essentially straight lines. In contrast, current paths in EIT are functions of an unknown resistivity distribution. This gives rise to a non-linear reconstruction problem.
The physiological basis for EIT, relies on the fact that biological tissues contain free charge carriers that permit them to act as relatively poor electrical conductors. This ability to conduct electricity varies substantially among the various body tissues. Some typical values for resistivity of biological tissues are disclosed in Table 1. The goal of EIT is to compute and display the spatial distribution of resistivity inside the body.
TABLE 1 ______________________________________ Resistivity Material (.rho.) ohm-cm ______________________________________ Blood 150 Plasma 63 Cerebrospinal Fluid 65 Urine 30 Skeletal muscle 300 Cardiac muscle 750 Lung 1275 Fat 2500 Copper 1.724 .times. 10.sup.-6 ______________________________________
The present invention is related to the subject matter of U.S. Pat. No. 4,920,490 issued to one of the co-inventors of the present application and incorporated here by reference.
This invention is also related to U.S. patent application Ser. No. 07/727,075 entitled A LAYER STRIPPING PROCESS FOR IMPEDANCE IMAGING, which is also incorporated here by reference and which discloses mathematical theories and manipulations which are useful in understanding the present invention. For additional disclosure concerning hardware useful in practicing the present invention, see U.S. patent application Ser. No. 07/734,591 entitled CURRENT PATTERNS FOR ELECTRICAL IMPEDANCE TOMOGRAPHY which is also incorporated here by reference.
Additionally, the present invention relates to trigonometric current patterns such as those described by Cheney et al., NOSER: An Algorithm For Solving the Inverse Conductivity Problem, 2 Int'l. J. Imaging Systems and Technology, 66-75 (1990).
The present invention also relates to Walsh function patterns such as those disclosed by U.S. patent application Ser. No. 07/591,615 entitled CURRENT PATTERNS FOR IMPEDANCE TOMOGRAPHY, now U.S. Pat. No. 5,272,624, which is incorporated herein by reference.